docs: def construct_transformed_regex(annotations):
      """
      construct a regex that matches possible fields in a transformed file
      annotations is a set of which keys in BARCODEINFO are present in the file
      """
      re_string = '.*'
      if "cellular" in annotations:
          re_string += ":CELL_(?P<CB>.*)"
      if "molecular" in annotations:
          re_string += ":UMI_(?P<MB>\w*)"
      if "sample" in annotations:
          re_string += ":SAMPLE_(?P<SB>\w*)"
      if re_string == ".*":
          logger.error("No annotation present on this file, aborting.")
          sys.exit(1)
      return re_string